The pressure sensor of such a capacitance type is used for various usages, for example, such as refrigerant pressure sensor for freezing, refrigeration, and air-conditioning equipment; hydraulic pressure sensors for water supplies and industrial pumps; vapor pressure sensor for steam boiler; air pressure/oil pressure sensor for air pressure/oil pressure industrial apparatus and pressure sensor for cars.
By the way, conventionally, as a device member, in which this kind of cavity is provided, for instance, as for the pressure sensor of the capacitance type, the producing method disclosed in Patent Document 1 (JP 2004-260187, A) is proposed.
FIG. 9-FIG. 13 is a schematic view showing the process of the method of producing the device member, in which such a conventional cavity is provided.
For instance, as shown in FIG. 9 (A), a device material 101 including the SOI (Silicon On Insulator) wafer, comprising a substrate member 102 including Si, an interlayer 104 including SiO2 formed on this substrate member 102, and an upper layer 106 including Si formed on the interlayer 104, is prepared.
Next, as shown in FIG. 9 (B), for instance, a photoresist layer 108 of the positive type is formed on the upper layer 106.
Moreover, as shown in FIG. 9 (C), through a photomask 110 having the predetermined pattern, the photoresist layer 108 is exposed, for instance, by using ultraviolet (UV) from the upper surface of this photoresist layer 108.
Thereafter, for instance, it is developed by the developer such as TMAH (Tetra Methyl Ammonium Hydroxide).
As a result, as shown in FIG. 10 (A), the photoresist of the exposed portion is removed, so that the photoresist layer 108 is remained in the form of the predetermined pattern.
Under such a condition, as shown in FIG. 10 (B), the photoresist layer 108 is used as a mask and the upper layer 106 is processed by using the method of Deep Reactive RIE (Deep Reactive Ion Etching).
That is to say, for instance, “the process in which the etching by SF6 and the passivation by C4F8 is alternately are performed”, that is called “Process of Bosch” is performed.
As a result, an opening portion 112, which is the same pattern as the photoresist layer 108 remained in the form of the predetermined pattern, is formed to the upper layer 106.
Next, as shown in FIG. 10 (C), all photoresist layers 108 are removed.
Moreover, as shown in FIG. 11 (A), for instance, the interlayer 104 is etched by using the HF gas, so that a gap 114 having the predetermined pattern is formed to the interlayer 104.
Moreover, as shown in FIG. 11(B), from the upper surface in the upper layer 106, for instance, metal such as aluminum, semiconductor such as polysilicon, or plastic such as parylene is deposited, so that a sealing layer 116 is formed.
As a result, a part of sealing layer 116 is entered into the opening portion 112 formed to upper layer 106 and is sealed, so that a cavity 118 is formed.
Next, as shown in FIG. 11 (C), for instance, photoresist layer 120 of the positive type is spin-coated on the upper surface of the sealing layer 116.
Thereafter, as shown in FIG. 12 (A), through a photomask 122 having the predetermined pattern, the photoresist layer 120 is exposed by, for instance, by using ultraviolet (UV) from the upper surface of this photoresist layer 120.
Thereafter, for instance, it is developed by the developer such as TMAH (Tetra Methyl Ammonium Hydroxide).
As a result, as shown in FIG. 12 (B), the photoresist of the exposed portion is removed, so that the photoresist layer 120 is remained in the form of the predetermined pattern.
Next, for instance, where the sealing layer 116 is aluminum, the aluminum etching fluid is used, or where the sealing layer 116 is a plastic, the O2 ashing by using the O2 plasma is used.
As a result, as shown in FIG. 12 (C), an unnecessary portion is removed from the sealing layer 116.
Thereafter, all photoresist layers 120 are removed.
Consequently, as shown in FIG. 13, a device member 100, in which the sealing layer 116 is formed in the form of the predetermined pattern, is obtained.
Though not shown in the drawing, in the device member 100, predetermined circuit, strain gauge (Strain gauge), and counter electrode, etc. are formed to the upper layer 106 according to the usage.
FIG. 15 is a schematic drawing that shows the schematic when the device member 100 manufactured from a conventional producing method is used as a pressure sensor of the capacitance type.
As shown in FIG. 15, in a pressure sensor 200, the upper layer 106, which is sealed by the above-mentioned sealing layer 116, is used as a diaphragm 202.
Moreover, the diaphragm 202 is used as a counter electrode 204.
In addition, the substrate member 102 consisting of Si is used as a counter electrode 206.
Moreover, as shown in FIG. 15 (A), and FIG. 15 (B), when the pressure of the cavity 118 is P1 and the pressure exerted to the diaphragm 202 from the outside is P2, it is a pressure sensor to detect this pressure difference.
Moreover, the cavity 118 is sealed to enter the state of the vacuum in the cavity 118.
As a result, a pressure sensor of the absolute pressure type is composed as a pressure standard room where pressure P1 in the cavity 118 becomes a vacuum.
By the way, capacity C of the general capacitor composed of two electrodes, which are separated mutually through the dielectric substance (insulator), is shown by a formula listed below.
                    C        =                  ɛ          ·                      S            d                                              [                  Formula          ⁢                                          ⁢          1                ]            
In this formula, ∈ is a dielectric constant of the dielectric substance (insulator) of interelectrode, and S is an area of the electrode, and d is a distance of interelectrode.
As it is clear from this formula, capacity C is proportional to dielectric constant ∈ of the dielectric substance (insulator) of interelectrode and to area S of the electrode respectively, and is in inverse proportion to distance d of interelectrode.
Similarly, the pressure sensor 200 of the capacitance type shown in FIG. 15 (A) and FIG. 15 (B) comprises the counter electrode 204 and counter electrode 206, which is two electrodes separated mutually through the interlayer 104 that is dielectric substance (insulator).
In this case, the counter electrode 204 that composes one of the electrodes is used as the diaphragm 202, which is displaced by the change in the pressure that is exerted to the counter electrode 204.
As a result, the distance d of the interelectrode between the counter electrode 204 that functions as the diaphragm. 202 and the counter electrode 206 that composes the other electrode is changed.
Therefore, as it is clear from the above formula, in the pressure sensor 200 of the capacitance type, the pressure, which is exerted to the counter electrode 204 that functions as the diaphragm 202, is changed.
As a result, the distance d of the interelectrode between the counter electrode 204 and the counter electrode 206 is changed. Accordingly, it can be considered as a variable capacity capacitor, in which capacity C is changed in inverse proportion to the distance d of interelectrode.
Moreover, as shown in FIG. 15 (A) and FIG. 15 (B), a part of a wire 208 is connected to the counter electrode 204 and apart of a wire 210 is connected to the counter electrode 206.
In addition, each other end of these wires 208, 210 are connected to the measuring control device not shown in the drawing.
As a result, through these wire 208 and wire 206, the voltage can be applied to the counter electrode 204 and the counter electrode 206 by the measuring control device.
As a result, as it is clear from the above formula, displacement of the counter electrode 204 by change in pressure which is exerted to the counter electrode 204 that functions as the diaphragm 202 can be perceived as a change in capacity C in inverse proportion to the distance d of interelectrode.
That is, change in the distance d of the interelectrode between the counter electrode 204 and the counter electrode 206 by displacement of the counter electrode 204, can be perceived as a change in capacity C in inverse proportion to the distance d of interelectrode.
Therefore, if the relation between this change in capacity C and the change of the pressure which is exerted to the counter electrode 204 that functions as the diaphragm 202 is previously recognized, the pressure, which is exerted to the counter electrode 204 at that time by measuring capacity C, can be perceived.
As a result, it can be used as a pressure sensor.
The pressure sensor by such a principle is generally called “Pressure sensor of the capacitance type”.
Moreover, though not shown in the drawing, as a pressure sensor, besides the pressure sensor of the capacitance type, the pressure sensor of the piezoresistance type is existed.
In this pressure sensor of the piezoresistance type, strain gauge (Strain gauge) is formed on the surface of the diaphragm and the diaphragm is transformed by pressure from the outside, so that the change in electric resistance according to the generated piezoresistance effect is converted into pressure.